It is disclosed in JP2004-183746A that a flow control valve is provided in a discharge-side flow path of an oil pump.
In JP2004-183746A, a spool which functions as a flow control valve includes an orifice and the spool moves according to a differential pressure between an upstream side and a downstream side of the orifice. The spool adjusts the opening area of a drain hole which is formed in a side surface of the spool and communicates with a suction passage of an oil pump. Nominally, the spool is biased to close the drain hole by a spring or the like. The spool moves to open the drain hole if the differential pressure between the pressure at the upstream side of the orifice and that at the downstream side of the orifice becomes larger than a biasing force of the spring.
By returning hydraulic oil discharged from the oil pump to the suction passage of the oil pump using such a flow control valve, the suction amount of the hydraulic oil mixed with air having a low compression ratio decreases and discharge efficiency of the oil pump is improved.
However, if more air is mixed into the hydraulic oil sucked into the oil pump from an oil pan, the discharge amount of the oil pump pulsates and the differential pressure also pulsates. As a result, even if the drain hole has to be essentially in a communicating state, the spool may move to close the drain hole by the biasing force of the spring, thereby bringing the drain hole into a non-communicating state, when the differential pressure becomes smaller due to pulsation.
In such a case, the hydraulic oil mixed with air having a low compression ratio is sucked into the oil pump and the volume of air in oil in the oil pump increases. Thus, the discharge amount of the oil pump further decreases and a necessary hydraulic pressure cannot be supplied, for example, in a transmission or the like.
Contrary to this, it is disclosed in JP2002-340160A that the amount of air mixed into hydraulic oil is calculated and the rotation speed of an electrical oil pump is increased in consideration of a reduction in discharge amount caused by air mixture.